Lennie needs George. Since his aunt died, he has no one to take care of him. Lennie is just a child in his mentality. He needs adult supervision. Also, it seems that George needs Lennie. He seems to truly care about Lennie. Although he gets aggravated with Lennie, as do most adults who deal with child-like misbehavior, George is as a parent to Lennie. Lennie will not survive without George.
In a memorable dialogue in chapter one, Lennie and George are discussing the mouse that Lennie has been caressing. George asks Lennie in his most aggravated voice:
'What you want of a dead mouse, anyways?'
'I could pet it with my thumb while we walked a long,' said Lennie.
This conversation shows Lennie's mentality. He is mentally challenged, and George is his only friend and companion. George is as family to Lennie. George cannot abandon Lennie at this point. He is as helpless as a child.
At one point, George admits even to Lennie that he is so much trouble with which to deal:
'God, you're a lot of trouble,' said George. 'I could get along so easy and so nice if I didn't have you on my tail. I could live so easy and maybe have a girl.'
In this conversation, it is clear that George would like to have a life of his own. So why does he put up with Lennie? Deep, down, George is an extremely caring man. He has taken Lennie into his care, and he is a man who follows through with his commitment either to Lennie or his aunt or both.
Truly, it is clear that George cares about Lennie. When he speaks of having someone to talk to, the reader can sense that George looks at Lennie as his lifetime companion:
"Guys like us, that work on ranches, are the loneliest guys in the world. They got no family. They don't belong no place...They ain't got nothing to look ahead to.'
Lennie was delighted. 'That's it--that's it. Now tell how it is with us.'
'With us, it ain't like that. We got a future. We got somebody to talk to that gives a damn about us.'
Clearly, this conversation proves that George cares and he will be lost without Lennie after he has to shoot him.